Vacuum pumps used to draw the gas stream from a process chamber are generally multistage pumps comprising a pair of drive shafts each supporting a plurality of rotors. A housing of the vacuum pump provides a stator within which the drive shafts and rotors rotate during use of the pump. The stator comprises a gas inlet, a gas outlet and a plurality of pumping chambers, with adjacent pumping chambers being separated by a transverse wall. A gas flow duct connects a chamber outlet from one pumping chamber to a chamber inlet of the adjacent, downstream pumping chamber. Each pumping chamber houses a pair of rotors such that there is a small clearance between the rotors, and between each rotor and an inner wall of the pumping chamber. The rotors typically have one of a Roots or Northey (“claw”) profile, and the profile of the rotors may change along the drive shafts.
During processes such as chemical vapour deposition, process gases are supplied to a process chamber to form a deposition layer on the surface of a substrate. As the residence time of the process gas in the chamber is relatively short, only a small proportion of the gas supplied to the chamber is consumed during the deposition process. The unconsumed process gas is subsequently pumped from the process chamber with one or more by-products from the process using a vacuum pump.
The gas stream pumped from the process chamber can contain species that may cause damage to the pump. For example, some deposition process generate solid particulates, such as SiO2 particulates, which are exhausted from the process chamber with the unconsumed process gases. In addition some deposition processes use vaporised liquid precursors, such as TEOS, which can condense and/or collect in the pump.
As another example, if the unconsumed process gas or by-product is condensable, condensation on low temperature surfaces within the vacuum line between the process chamber and the vacuum pump, or within the vacuum pump itself, can result in significant amounts of powder or dust passing through the pump.
Any solid or liquid material passing through a twin-shafted vacuum pump is forced between the rotors of the pump, and it has been observed that, over time, this can result in damage to the rotors or, in some cases, cause the rotors to hydraulically lock. For Northey rotors, the damage usually manifests as swelling of the edges of the rotors, which can reduce the size of the clearances between the rotors and between the rotors and the stator. This could compromise the future reliability of the pump, particularly if the pump is operated at higher temperatures as the thermal expansion of the rotors relative to the stator could now lead to contact between the rotors, and/or between a rotor and the stator.